Exercise set

Landfill Disposal, Leachate, Gas, and Airspace Exercises

Solved landfill engineering exercises for leachate storage, head control, airspace life, compaction, gas capture, flare downtime and release gates.

Branch: Environmental Engineering
Content: Exercise set
Updated: Jul 03, 2026
Revision: v1.0.0 · reviewed

These exercises focus on landfill disposal controls rather than the whole solid-waste system. They cover leachate storage, treatment margin, head-on-liner, airspace life, compaction, daily cover, methane generation, gas collection, flare downtime, condensate, pump availability and release evidence.

Assume simplified screening calculations unless an exercise states otherwise. Field decisions require site permit limits, liner design, leachate head sensors, pump logs, settlement surveys, gas-header data, surface-emission scans, flare or engine records and corrective-action evidence.

Release Evidence Notes

Landfill evidence must preserve both mass and pathway. Waste tonnage affects airspace and gas; water balance affects leachate head and hauling; gas capture affects odor, explosion risk, greenhouse release and energy recovery.

Leachate evidence should state generation rate, storage volume, treatment or hauling capacity, starting inventory, pump status and head-on-liner margin. Wet-weather periods can control the decision even when annual averages look acceptable.

Gas evidence should state methane generation basis, collection efficiency, header capacity, condensate management, flare availability and surface-emission check results. A captured gas number is weak if downtime, bypass and wellfield balance are not shown.

Engineering Boundary Notes

These calculations do not replace landfill design, permit modeling, groundwater monitoring, gas migration review, surface-emission monitoring or closure engineering. They are screening exercises for disposal operations and release packages.

Common Release Mistakes

using average leachate generation when the release decision is controlled by a storm week;
counting gross airspace without daily cover, intermediate cover, settlement or unusable geometry;
treating landfill gas power as proof of gas control without collection and flare evidence;
ignoring condensate or header pressure limits that prevent gas collection;
closing a release gate without matching tonnage, liquid, gas and inspection records.

Scenario Map

Scenario	Exercises	Primary check	Engineering decision
Leachate control	1, 2, 3, 14, 15	storage, treatment, head, pump availability and hauling	Decide whether liquids control is credible.
Airspace and density	4, 5, 6, 7, 16	remaining life, compaction, cover use and settlement	Decide whether disposal capacity is defensible.
Gas control	8, 9, 10, 11, 12, 13	methane generation, collection headroom, flare downtime and emissions	Decide whether gas risk is controlled.
Release gate	17, 18	evidence completeness and all-of release	Decide whether the landfill package can close.

Exercise 1: Wet-Weather Leachate Storage

A landfill cell generates $68\ \text{m}^3/\text{d}$ of leachate during a storm period. Treatment and hauling capacity is $45\ \text{m}^3/\text{d}$ for $5$ days. Compute required temporary storage for the excess leachate.

Solution

Q_{ex}=68-45=23\ \text{m}^3/\text{d}

V_{ex}=23(5)=115\ \text{m}^3

Engineering Comment

At least $115\ \text{m}^3$ of usable storage is needed before freeboard or contingency volume is added.

Plausibility Check

The excess is about one third of daily generation; over five days that gives a storage need a little above one hundred cubic meters.

Exercise 2: Leachate Treatment Capacity Margin

Average leachate flow is $52\ \text{m}^3/\text{d}$ . The treatment skid can process $70\ \text{m}^3/\text{d}$ . Compute the capacity margin.

Solution

\text{margin}=\dfrac{70-52}{52}=0.346

\text{margin}=34.6\%

Engineering Comment

A $34.6\%$ margin may be adequate for average operation, but storm inflow and pump downtime still need separate checks.

Plausibility Check

The spare capacity is $18\ \text{m}^3/\text{d}$ , roughly one third of the average load.

Exercise 3: Head-on-Liner Margin

Measured leachate head is $0.26\ \text{m}$ . The permit action level is $0.30\ \text{m}$ . Compute the remaining head margin.

Solution

h_m=0.30-0.26=0.04\ \text{m}

h_m=40\ \text{mm}

Engineering Comment

Only $40\ \text{mm}$ remains. That is a narrow operating margin if the sensor uncertainty, pump cycling band or storm response is comparable.

Plausibility Check

The measured head is close to the limit, so the margin should be small.

Exercise 4: Remaining Landfill Airspace Life

Remaining permitted airspace is $420000\ \text{m}^3$ . Annual accepted waste is $150000\ \text{t/yr}$ and in-place density is $0.82\ \text{t/m}^3$ . Estimate remaining life, ignoring cover and settlement.

Solution

Annual volume consumed is:

V_y=\dfrac{150000}{0.82}=182927\ \text{m}^3/\text{yr}

Remaining life:

L=\dfrac{420000}{182927}=2.30\ \text{yr}

Engineering Comment

This is a gross screen. Daily cover, intermediate cover, slope geometry and settlement can change the usable life.

Plausibility Check

The site consumes almost $183000\ \text{m}^3$ per year, so a little more than two years is reasonable.

Exercise 5: Compaction Density Improvement

Waste placement increases from $0.74$ to $0.86\ \text{t/m}^3$ . For $120000\ \text{t/yr}$ , compute annual airspace saved.

Solution

Before improvement:

V_1=\dfrac{120000}{0.74}=162162\ \text{m}^3

After improvement:

V_2=\dfrac{120000}{0.86}=139535\ \text{m}^3

V_s=162162-139535=22627\ \text{m}^3/\text{yr}

Engineering Comment

Compaction is an airspace control, not only an equipment productivity measure.

Plausibility Check

The density gain is material, so saving tens of thousands of cubic meters per year is plausible.

Exercise 6: Daily Cover Airspace Penalty

A cell receives $480\ \text{m}^3/\text{d}$ of compacted waste. Daily cover adds $12\%$ additional volume. Compute total daily airspace consumption.

Solution

V_t=480(1+0.12)=537.6\ \text{m}^3/\text{d}

Engineering Comment

Cover volume should be included in airspace accounting unless an approved alternative daily cover reduces the penalty.

Plausibility Check

A $12\%$ addition on about $500\ \text{m}^3/\text{d}$ should add about $60\ \text{m}^3/\text{d}$ .

Exercise 7: Diversion Effect on Cell Life

A cell has $250000\ \text{m}^3$ of usable airspace. Baseline disposal is $95000\ \text{m}^3/\text{yr}$ . A diversion program reduces disposal by $14\%$ . Estimate new life.

Solution

New annual consumption:

V_y=95000(1-0.14)=81700\ \text{m}^3/\text{yr}

Cell life:

L=\dfrac{250000}{81700}=3.06\ \text{yr}

Engineering Comment

Diversion should be credited only when residual mass and destination evidence are traceable.

Plausibility Check

Reducing annual consumption from $95000$ to $81700\ \text{m}^3$ stretches life from $2.63$ to just over $3$ years.

Exercise 8: Methane Generation Screen

Estimated landfill gas flow is $980\ \text{m}^3/\text{h}$ with $48\%$ methane. Compute methane flow.

Solution

Q_{CH4}=0.48(980)=470.4\ \text{m}^3/\text{h}

Engineering Comment

The methane fraction matters for energy, flare temperature and explosion-risk evaluation.

Plausibility Check

About half of the total gas is methane, so methane flow should be close to $500\ \text{m}^3/\text{h}$ .

Exercise 9: Gas Collection Headroom

Measured landfill gas flow is $980\ \text{m}^3/\text{h}$ . The header fan can handle $1250\ \text{m}^3/\text{h}$ . Compute headroom.

Solution

\text{headroom}=\dfrac{1250-980}{980}=0.276

\text{headroom}=27.6\%

Engineering Comment

Header headroom is useful only if individual wells, condensate traps and flare capacity can also accept the added flow.

Plausibility Check

The spare flow is $270\ \text{m}^3/\text{h}$ , a little over one quarter of the current flow.

Exercise 10: Flare Downtime Methane Venting

During a flare outage, $470\ \text{m}^3/\text{h}$ of methane is not destroyed for $6$ hours. Estimate the undestroyed methane volume.

Solution

V_{CH4}=470(6)=2820\ \text{m}^3

Engineering Comment

The release record should distinguish planned outage, emergency outage, bypass route and whether gas was routed to another control device.

Plausibility Check

Several hundred cubic meters per hour over several hours gives several thousand cubic meters.

Exercise 11: Condensate Knockout Capacity

A gas header produces $0.18\ \text{m}^3/\text{h}$ of condensate. The knockout pot has $1.2\ \text{m}^3$ of usable volume. Estimate time to fill if drainage is blocked.

Solution

t=\dfrac{1.2}{0.18}=6.67\ \text{h}

Engineering Comment

Condensate can become the limiting gas-control failure mode even when fan capacity looks adequate.

Plausibility Check

At less than a quarter cubic meter per hour, a one cubic meter vessel fills over several hours, not minutes.

Exercise 12: Landfill Gas Engine Power

Captured methane energy input is $4.7\ \text{MW}_{th}$ . Engine electrical efficiency is $34\%$ . Compute electrical output.

Solution

P_e=0.34(4.7)=1.60\ \text{MW}

Engineering Comment

Electrical output is not gas-control proof. A flare or backup path is still needed when the engine is unavailable.

Plausibility Check

One third of about $5\ \text{MW}_{th}$ is about $1.7\ \text{MW}$ .

Exercise 13: Surface Emission Exceedance Rate

A surface-emission scan checks $160$ points. Twelve points exceed the action level. Compute exceedance rate.

Solution

r=\dfrac{12}{160}=0.075=7.5\%

Engineering Comment

The rate should trigger wellfield adjustment and cover inspection if it exceeds the site’s operating criterion.

Plausibility Check

Twelve out of one hundred sixty is less than one tenth, so $7.5\%$ is plausible.

Exercise 14: Leachate Pump Availability

Two leachate pumps are arranged one duty and one standby. Each pump availability is $0.93$ . Estimate availability of at least one pump, assuming independent failures.

Solution

Probability both fail:

P_f=(1-0.93)^2=0.0049

Availability of at least one pump:

A=1-0.0049=0.9951

Engineering Comment

Independence is a strong assumption. Shared power, level controls and clogged sumps can defeat redundancy.

Plausibility Check

Two standby-capable pumps should be much more available than one pump if failures are independent.

Exercise 15: Leachate Hauling Slot Check

Excess leachate is $115\ \text{m}^3$ . Each tanker carries $23\ \text{m}^3$ . Compute required tanker loads.

Solution

N=\dfrac{115}{23}=5

Engineering Comment

The calculation closes only if the receiving plant, access road and loading station can support the five trips in the required window.

Plausibility Check

Five equal tankers at $23\ \text{m}^3$ each exactly remove $115\ \text{m}^3$ .

Exercise 16: Settlement Airspace Gain

Surveyed settlement creates $18000\ \text{m}^3$ of additional apparent airspace. If only $70\%$ is usable because of grading constraints, compute usable gain.

Solution

V_u=0.70(18000)=12600\ \text{m}^3

Engineering Comment

Settlement should be reconciled with grades, stormwater controls and final cover requirements before being counted as disposal capacity.

Plausibility Check

Seventy percent of $18000$ is a little over $12000\ \text{m}^3$ .

Exercise 17: Landfill Evidence Closure Score

A landfill release checklist has $8$ required evidence items: leachate generation, storage, head trend, pump status, airspace survey, waste tonnage, gas collection and surface-emission scan. Six are complete. Compute completion.

Solution

C=\dfrac{6}{8}=0.75=75\%

Engineering Comment

A $75\%$ record is not release-ready if the missing items are pathway controls such as head trend or gas scan.

Plausibility Check

Six of eight items is three quarters.

Exercise 18: Landfill Release Gate

A release gate requires: leachate head below action level, storage margin above $100\ \text{m}^3$ , gas collection headroom above $20\%$ , no unresolved surface-emission action, and evidence completion at least $90\%$ . Current values are $0.26\ \text{m}$ versus $0.30\ \text{m}$ action level, $115\ \text{m}^3$ storage margin, $27.6\%$ gas headroom, one unresolved surface-emission action and $75\%$ evidence completion. Decide release status.

Solution

Head passes, storage passes and gas headroom passes. Surface-emission action fails, and evidence completion fails:

75\%<90\%

Release status:

\text{hold}

Engineering Comment

The correct decision is to hold release. Passing hydraulic and gas-capacity numbers does not close unresolved field evidence.

Plausibility Check

An all-of gate fails when any required condition fails; here two conditions fail.

Validation Package Checklist

Leachate generation, storage, treatment or hauling capacity, head trend and pump status are on the same time basis.
Airspace calculations include density, cover, settlement, survey limits and usable geometry.
Gas calculations include methane fraction, collection capacity, flare or engine availability, condensate and surface checks.
Release decisions preserve unresolved corrective actions instead of hiding them inside average performance.

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